1. Field of the Invention
The present invention relates to an interference measurement apparatus capable of noncontact detection of position change information of an object. It relates particularly, though not exclusively, to an interference measurement apparatus suitable for detecting absolute position shift information by adding a position as an origin when detecting displacement information.
2. Description of the Related Art
For a measurement apparatus capable of measuring length with accuracy and resolution of the order of nanometers, a laser interference measurement apparatus, such as a Michelson interference apparatus using a laser beam, is often used.
Since an interference wave generated by the displacement of the object being measured is a sine wave, a laser interference measurement apparatus can count the wave number of the sine wave and detect the phase of the sine wave to measure the amount of displacement to an accuracy and resolution of the order of nanometers.
Japanese Patent Laid-Open No. 2001-76325 describes an apparatus configured to obtain displacement information (position information) of a measurement plane by using an interference signal obtained by a Michelson interference apparatus.
The displacement detection apparatus described in Japanese Patent Laid-Open No. 2001-76325 uses optical interference to detect displacement information of a measurement plane provided on a magnetic head arm.
More specifically, a probe instruction arm for alignment is provided coaxial to the rotational shaft of the magnetic head arm.
On the probe instruction arm, an optical position detection sensor unit configured to detect whether or not the end surface (i.e., measurement plane) of the magnetic head arm is at a desirable position.
The optical position detection sensor unit includes a detection system configured to align the side surface of the magnetic head arm by using an interference signal. The detection system includes a quarter wave plate, a phase diffraction grating, and light-receiving element.
In addition, a focus detection optical system including a polarizing plate, a condenser lens, and a quadripartite sensor is provided to detect position information on the measurement plane in the direction of the optical axis.
The output signal of an interference measurement apparatus obtained by carrying out laser interference is a sine wave. Thus, measurement can be carried out at a resolution on the order of nanometers based on the wave number and phase. However, the absolute position cannot be determined merely by displacement. Therefore, to obtain absolute displacement information of the object being measured, an additional reference point (i.e., origin) must be provided and the relative position of the object with respect to this reference point must be determined.
In this case, to determine the accurate absolute position, the reference point (i.e., origin) must be provided at an accuracy surpassing the wavelength (0.4 μm for the example above) of the output sine wave obtained by carrying out laser interference.
The focus detection optical system of the displacement detection apparatus according to Japanese Patent Laid-Open No. 2001-76325 determines the origin of the measurement plane by detecting the ratio of the amounts of light incident upon the sensors of the quadripartite sensor. Thus, interference signals are not used.
For this reason, it is difficult to detect position information of the measurement plane at an accuracy of the order of the wavelength.